Monolithically integrated inductor devices formed on semiconductor substrates are well known. An integrated inductor device employs a conductive loop (having one or more turns) fabricated from one or more appropriate metal interconnect layers on the semiconductor substrate. In some applications, a solid or continuous ground shield resides below and/or above the conductive inductor loop to electrically isolate the inductor from noisy electromagnetic fields that might be generated by other nearby devices in the shared substrate or conductive elements in the device package.
Solid or continuous inductor ground shields may be undesirable in certain applications, such as integrated inductors for resonant oscillators. In such applications, magnetically induced eddy current loops will be generated in the solid shields. The resulting eddy current loops in the solid shield produce magnetic flux that, in accordance with Lenz's law, counters the magnetic flux produced by the inductor, thereby reducing the overall effective inductance of the inductor device. To address this reduction in effective inductance for an allocated inductor area, patterned or slotted shields are commonly used. A patterned ground shield typically includes metal “fingers” that are arranged to be orthogonal to the direction of the inductor loop so as to inhibit the formation of eddy current loops. As a result, these fingers are magnetically transparent yet provide adequate electric field isolation of the inductor from offending substrate noise.
Moreover, integrated inductors used in circuits such as resonant oscillators are usually not tunable (at least within a practical range of inductance)—frequency tuning is typically achieved only with capacitive tuning. Wide frequency tuning in oscillators is highly desirable for synthesizing multiple frequency targets without the area cost of additional circuitry. Accordingly, compared to resonant oscillators, ring oscillators typically exhibit wider tuning range, but their relatively noisier or higher jitter performance characteristic renders them unusable in meeting more stringent frequency or clock synthesis specifications.
Accordingly, it is desirable to have a tunable, integrated inductor device that has a usable and practical inductance range. Furthermore, other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.